1. Field of the Invention
The present invention relates to a developing apparatus used for an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus.
2. Related Background Art
In an image forming apparatus using an electrophotographic system, an electrostatic latent image is formed on an image bearing body, which is developed into a toner image to be visualized, and then, the toner image is transferred onto a transfer material to obtain an image.
In general, such developing methods include a one-component developing method using a one-component developer made of only a magnetic toner, and a two-component developing method using a two-component developer made of a magnetic particle (magnetic carrier) and a toner. The one-component developing method has such merits that the structure of a developing apparatus is simple and maintenance is easy, so that a variety of such developing apparatuses have been proposed and have been put into practical use.
There has also been proposed a developing method by a nonmagnetic, one-component developer made of only a nonmagnetic toner as disclosed in Japanese Patent Application Laid-open No. 58-116559, and such a developing apparatus is put into practical use that it can cope with colorization of an image in recent years and can obtain a high quality color image, while it is inexpensive and small.
FIG. 8 shows an example of a nonmagnetic, one-component developing apparatus. As shown in FIG. 8, in the developing apparatus, a developing roller 1 made of an elastic roller is set as a developer bearing body in a developer container 3 containing a nonmagnetic toner of a nonmagnetic, one-component developer, and a toner supplying roller 14 as a developer supplying member and a regulating blade 13 as a developer regulating member are set to abut against the developing roller 1.
The developing roller 1 includes a solid rubber-like elastic layer having a hardness of about 30 to 70.degree. in asker C hardness, and is rotatively driven in a direction indicated by an arrow while being in contact with the photosensitive drum 11. A coating material for more effectively performing electric charge application to the toner may be provided on the surface of the developing roller 1. Incidentally, since the toner is borne on the surface of the developing roller 1, the developing roller 1 is not in direct contact with the photosensitive drum 11. However, in this specification, the recitation "the developing roller 1 is in contact with the photosensitive drum 11" includes such a state.
The toner supplying roller 14 is formed by coating the outer surface of a core metal 14a of SUS or the like with an elastic member 14b of urethane foam or the like. The roller 14 rotates while rubbing or sliding frictionally against the developing roller 1, so that the nonmagnetic toner contained in the developer container 3 is supplied to the surface of the developing roller 1. The roller 14 has also a function to scrape the nonmagnetic toner, which has not contributed to development at a developing portion opposite to the photosensitive drum 11 and has been returned to the developer container 3 with the rotation of the developing roller 1, from the surface of the developing roller 1.
The regulating blade 13 is made of a metal plate of, for example, a stainless metal plate with a thickness of about 0.1 to 0.5 mm, and is disposed to contact with the surface of the developing roller 1 at a pressure (linear pressure) of about 5 to 100 g/cm, so that the toner borne on the developing roller 1 is regulated to form a thin layer of toner, and a friction charging electric charge is applied to the toner.
By the foregoing structure, the developing apparatus makes it possible to excellently form the thin layer of the charged nonmagnetic toner on the developing roller 1 and to supply the toner for development of an electrostatic latent image formed on the surface of the photosensitive drum 11, so that excellent development of the latent image becomes possible.
However, in the foregoing nonmagnetic, one-component developing apparatus, the latent image is developed in the state where the developing roller 1 abuts against the photosensitive drum 11 and the toner is rubbed. Also in the developer container 3, the toner supplying roller 14 abuts against the developing roller 1, so that the toner is rubbed and is supplied to the developing roller 1, and then the toner on the developing roller 1 is collected. Further, application of the electric charge to the toner is mainly performed by contact friction when the toner passes through the regulating blade 13.
From these, in the nonmagnetic, one-component developing apparatus, before a toner in the developer container 3 is used for development at the photosensitive drum 11, an extremely large mechanical load is applied to the toner, so that damage of the toner is very serious as compared with other developing methods.
Further, depending upon the arrangement position or rotation direction of the toner supplying roller 14, there may be a case where the toner, which has not contributed to development, can not be completely collected so that the toner remains on the developing roller 1. The toner remaining on the developing roller 1 again passes through the regulating blade 13 and is carried to the development region. If such re-carrying is repeatedly performed, an external additive and the like for controlling a charging amount or fluidity of the toner is embedded into the toner by mechanical friction or heat accumulation received at each time, and the toner is deteriorated, so that desired charging characteristics and fluidity cannot be obtained.
Such a deteriorated toner causes many troubles in an image forming process. For example, in the case where the deteriorated toner is used for development, suitable developing characteristics cannot be obtained, so that there is a case where development failure may occur, a case where a void is produced in an image at the time of transfer, or the like. Further, supply of a new toner to the developing roller 1 is blocked, and an application amount of the toner onto the developing roller 1 may be lowered, so that density failure may be caused. When the deteriorated toner is fused to a nip portion between the regulating blade 13 and the developing roller 1 or to the surface of the developing roller 1, a poor toner coat such as a streak occurs on the developing roller 1, and electric charge application to the toner newly supplied to the developing roller 1 is hindered. By this, a noncharged toner is fed to the development region, so that bad quality of an image, such as fog and unevenness, may occur.
Besides, in the nonmagnetic, one-component developing apparatus, not only a load applied to the toner is large, but also a load applied to the developing apparatus itself is large. For example, with respect to the toner supplying roller 14, in the case where it is formed of the sponge type roller as described above, if rubbing against the developing roller 1 continues for a long time, wear occurs and damage of the toner supplying roller 14 itself, clogging of the toner, and the like, so that excellent supply and collection of a toner becomes impossible.
In this way, although the nonmagnetic, one-component developing apparatus has a simple structure and an excellent developed image can be obtained. The load applied to the toner and the apparatus is high, so that as compared with a magnetic one-component developing apparatus or a two-component developing apparatus, long stability is poor. Thus, the nonmagnetic, one-component is mainly used for a cartridge type developing apparatus in which the whole developing apparatus is replaced when the toner is consumed, and it is seldom used with a developing apparatus of a type wherein the toner is replenished.
Besides, in recent years, for the purpose of reducing consumed electric power, realization of a toner which can be fixed at a lower temperature is desired, and a developing apparatus having a developing process of low stress corresponding to the toner for low temperature fixation is demanded.